Styrene (phenylethylene, vinylbenzene) is commonly produced in a two-step process. First, ethylbenzene (EB) is formed by alkylating benzene, by transalkylating polyethylbenzenes (PEBs), or by both. Then, the EB is dehydrogenated to produce styrene. Styrene is an important monomer used in the manufacture of many plastics.
In the first step, benzene is alkylated with an ethylating agent such as ethylene to form EB. Diethylbenzenes (DEBs), triethylbenzenes (TEBs), and other heavier PEBs are also formed. To maximize EB formation, the PEBs are usually transalkylated with benzene to form more EB. When both alkylation and transalkylation are used, two separate reactors, each with its own catalyst, are often employed. Both the alkylation and transalkylation effluents flow to a distillation train, which recovers benzene, EB, and the light PEBs (DEBs and TEBs) as distillates in three distillation columns in series. These columns are called the benzene column, the EB column, and the PEB column. Benzene distillate from the benzene column is recycled to the alkylation and transalkylation reactors, and the light PEBs distillate is recycled to the transalkylation reactor. Examples of distillation trains for separating EB produced by alkylation and transalkylation are described in U.S. Pat. Nos. 4,169,111 and 4,891,458; PCT Publication WO 96/20148; and Catalysis of Organic Reactions, edited by W. R. Moser, Marcel Dekker, Inc., New York, USA, 1981, at pages 39-50.
In the second step, the EB is dehydrogenated to styrene in the presence of steam, which supplies the sensible heat needed for the endothermic reaction. Byproducts of this dehydrogenation reaction include benzene, toluene, and heavies (tar). The separation of the dehydrogenation effluent (which the prior art sometimes refers to as “crude styrene”) to recover styrene from steam, unreacted EB, and the byproducts is reasonably straightforward using three or four distillation columns.
In one distillation scheme, a first column separates or splits the effluent into EB and lighter components in its overhead and styrene and heavier components in its bottoms. A second column separates the first column's overhead into benzene and toluene in its overhead and EB in its bottoms for recycling EB to the dehydrogenation reactor. A third column separates the first column's bottoms into styrene product in its overhead and heavies (tar) in its bottom. U.S. Pat. No. 3,409,689, the teachings of which are hereby incorporated herein by reference, describes this scheme. According to this patent, the hydrocarbonaceous phase removed from the phase separation section passes into an effluent splitter distillation column. This first column separates or splits the effluent into EB and lighter components in an overhead stream and into styrene and heavier components in its bottoms stream. This overhead stream passes to a second distillation column called an EB recovery column to produce an overhead stream containing benzene and toluene and a bottom product containing EB. The EB product is recycled. The bottom stream of the effluent splitter column flows to styrene distillation column to produce purified styrene as an overhead stream and a bottom stream containing heavies.
Another distillation flow scheme recovers benzene-toluene, EB, and styrene as distillates in three distillation columns in series, as described in U.S. Pat. No. 3,525,776, the teachings of which are hereby incorporated herein by reference. In this patent, the hydrocarbonaceous phase removed from the phase separation section passes into a benzene-toluene recovery distillation column. This first column operates at a subatmospheric pressure to allow its operation at lower temperatures and hence reduce the rate of styrene polymerization. Within the benzene-toluene recovery column a separation of benzene and toluene from the effluent occurs to produce an overhead stream which is substantially free of styrene and EB. This overhead stream preferably contains at least 95 mol-% benzene and toluene. The bottoms stream of the benzene-toluene recovery column passes into an EB recovery distillation column from which EB is removed as an overhead product and recycled. The bottoms stream of this EB recovery column then passes to a styrene column to produce purified styrene as an overhead stream and a bottom stream containing heavies.
Using either distillation flow scheme, a fourth column can further purify the styrene product. For further information and examples of these distillation trains, see U.S. Pat. No. 4,252,615 (Watson); Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A25, VCH Publishers, New York, USA, 1994, at pages 329-344, and especially at pages 334-5; Encyclopedia of Chemical Processing and Design, Vol. 55, Marcel Dekker, Inc., New York, USA, 1996, at pages 197-217, and especially at pages 203-205; the technical sheet entitled “Lummus/UOP Classic SM Process,” UOP LLC, Des Plaines, Ill., USA, 1997; and the technical sheet, “Ethylbenzene/‘Classic’ Styrene Monomer,” ABB Lummus Global, Bloomfield, N.J., USA, Mar. 29, 2001.
Typically the only benzene present in the dehydrogenation zone is the relatively small amount that is formed as a byproduct of the reactions that take place during EB dehydrogenation. This benzene is subsequently recovered in the benzene-toluene fraction. The quantity of byproduct benzene is usually considered to be so commercially insignificant that operators of some styrene plants simply reject the entire benzene-toluene fraction from the plant for some other use or to disposal. However, operators of other styrene plants may either lack an alternative use for this fraction or may wish to avoid the disposal costs. Also, operators may introduce benzene to the dehydrogenation zone as described in U.S. Pat. Nos. 3,409,689 and 3,525,776, and so more benzene than just byproduct benzene may be present in the benzene-toluene fraction. These styrene plant operators distill the benzene (including byproduct benzene) from the benzene-toluene fraction and then recycle it to previously mentioned distillation train that is used for separating the alkylation and transalkylation effluents. In some cases, this benzene from the dehydrogenation zone is introduced into the first column (benzene column) of the distillation train, where it is recovered in the benzene-containing overhead stream. In other cases, it is combined directly with some or all the overhead stream of the benzene column.
Since benzene is, of course, a major feedstock for both the alkylation and transalkylation reactors, the benzene column of this distillation train or its overhead stream is also the destination for other, much larger flows of benzene. Because benzene is often supplied to these reactors in a large molar excess, the alkylation and transalkylation reactor effluents each carry a major flow of benzene to the benzene column. In addition, makeup benzene enters the benzene column or into that column's overhead. The effect of combining these various flows of benzene in the benzene column or its overhead is that the distillation train for the alkylation and transalkylation reactor effluents produces what amounts to a relatively homogeneous stream of benzene for recycling to the alkylation and transalkylation reactors. The relatively small amount of benzene arriving from the dehydrogenation zone is significantly diluted in this much larger stream.
That flow of recycle benzene, in turn, is split into two portions, with one portion passing to the alkylation reactor and the other to the transalkylation reactor. The split of benzene between the alkylation and transalkylation reactors depends on the operating conditions of the two zones, but generally less than 30% of the benzene recovered from the dehydrogenation zone passes to the transalkylation reactor. Except for the capital and operating costs of the additional distillation column needed to separate the benzene-toluene fraction, persons of ordinary skill in the art of styrene plants have viewed recycling the benzene from the dehydrogenation zone in this manner as technically-sound, as economically-acceptable, and as having no significant deleterious effect on the process.
While the distillation flow schemes for separating the alkylation, transalkylation, and dehydrogenation effluents are reasonably straightforward, it is well known that difficulties arise in distilling the dehydrogenation effluent. One difficulty is corrosion, since acidic aqueous solutions tend to condense in the cooler overhead sections of the distillation columns. Another difficulty is styrene polymerization in the hotter sections (typically from about 90° C. to about 150° C. (194° F. to 302° F.)) of the columns, since styrene tends to autopolymerize.
To prevent corrosion and polymerization, small amounts of inhibitors are added to the dehydrogenation effluent and/or the distillation train. The optimum choice of inhibitor(s) involves weighing many factors besides inhibition effectiveness, including cost, availability, volatility, toxicity, thermal stability, solubility, viscosity, whether oxygen is present, and the nature of the resultant residue. While individual styrene plants may use different inhibitors, most if not all styrene plants today use at least one inhibitor. The exact chemical compositions of inhibitors in commercial use today are not widely known, since the commercial suppliers of these inhibitors tend to keep this information secret. However, since the chemistry of both corrosion and polymerization is well understood, certain general characteristics of these inhibitors are well known to persons of ordinary skill in the art.
As concerns corrosion inhibitors, it is generally believed that many may be nitrogen compounds. These nitrogen compounds are believed to possibly include primary, secondary, and tertiary amines. One or more hydrogen atoms of the amine may be replaced with one or more alkyl groups or hydroxy alkyl groups. It is believed ethanolamine and diethanolamine may be in use. Other nitrogen compounds thought to possibly be in use include diamines and triamines. Each nitrogen atom of the diamines and triamines may be linked to one or more hydrogen atoms, alkyl groups, or hydroxy alkyl groups. The possible diamines may include ethylene diamine. Other possible classes of nitrogen compounds that may be in use include amides, N-(acyloxy)-alkane amines, dihydro-1-alkyl-N-substituted imidazoles (e.g., dihydro-1-alkyl-N-hydroxyalkane-imidazoles and dihydro-1-alkyl-N-aminoalkane-imidazoles), trialkylaminium dialkyl phosphates, and trialkylaminium alkyl hydrogen phosphates. Besides the corrosion inhibitors listed here, others may be in use.
As for polymerization inhibitors, the previously-mentioned reference in Ullmann's Encyclopedia of Industrial Chemistry states that at one time sulfur was used, but many new inhibitors are aromatic compounds that have amino, nitro, or hydroxy groups, including phenylenediamines, dinitrophenols, and dinitrocresols. It is believed that aromatic compounds that have nitroso groups are also in use as polymerization inhibitors. The “Background of the Invention” section of U.S. Pat. No. 6,395,943 B1 (Kurek and Frame) summarizes the extensive art disclosing a variety of compounds which are claimed to inhibit polymerization. These include N,N-nitroso-methylaniline; N-nitrosodiphenyl amine in combination with dinitro-o-cresol; N-nitroso aniline derivatives; a mixture of dinitro-p-cresol and N-nitroso-diphenyl amine; alkyl substituted p-nitroso phenol in combination with p-nitroso phenol; N-nitrosophenyl-hydroxylamine plus hydroquinone monomethyl ether; a phenylene-diamine compound plus a hydroxyalkylhydroxyl-amine compound; 1-oxy-2,2,6,6-tetramethylpiperidine plus an aromatic nitro compound; a phenylenediamine compound plus a hindered phenol compound; the reaction product of a C9-C20 alkyl phenol with sulfuric and nitric acid and optionally an aryl or alkyl-substituted phenylenediamine; 3,5-di-tert-butyl-4-hydroxy-N,N-dimethyl benzyl amine; 4-acetylamino-2,2,6,6-tetramethyl piperidine N-oxyl in combination with 4-nitroso phenol; phosphite compounds, nitrosoamine compounds or phenol compounds; the ammonium salt of N-nitrosophenyl hydroxylamine; nitrosophenols plus dicyclohexyl-ammonium nitrate; substituted nitrosobenzene; p-nitroso phenol plus p-t-butyl catechol; N-nitroso compound, e.g., N-nitroso-diphenylamine and a catechol, e.g., p-t-butylcatechol; and N-nitroso derivates of unsubstituted or dialkyl substituted phenylenediamine. U.S. Pat. No. 6,395,943 B1 itself discloses a mixture of at least one nitroso compound such as N,N′-di-2-butyl-N,N′-dinitroso-1,4-diaminobenzene and a dinitrophenol compound such as dinitrocresol, and optionally a stabilizer compound such as an N,N′-dialkyl substituted 1,4-diaminobenzene.
When corrosion inhibitors are used, they are typically added into the upper portions or overhead sections of the distillation columns. When polymerization inhibitors are used, they are typically added to distillation mixtures containing styrene or distillation columns processing styrene. Typically, any polymerization inhibitor is added to the dehydrogenation effluent stream passing to the first distillation column in the dehydrogenation separation section. When inhibitors are used, generally less than 30% of the inhibitors recovered from the dehydrogenation zone passes to the transalkylation reactor. For further information on the use of specific polymerization inhibitors in styrene production, see Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, 2853-2858 (1988); U.S. Pat. No. 5,869,717 (Frame et al.) and the previously mentioned references of U.S. Pat. No. 4,252,615, Ullmann's Encyclopedia of Industrial Chemistry, Encyclopedia of Chemical Processing and Design, the UOP LLC technical sheet, and the ABB Lummus Global technical sheet.
In order to be commercially profitable, industrial styrene plants must operate uninterrupted and for extended periods of time. Shutdowns must be minimized. One obstacle to maintaining continuous production is that the alkylation catalyst deactivates over time. It is known, of course, that the rate of catalyst deactivation can be decreased somewhat by operating at a high benzene/olefin molar ratio in alkylation, and that deactivated catalyst can be reactivated to some extent by contacting the catalyst with benzene in a regeneration step. However, it is also known that some catalyst deactivation occurs that can be neither slowed by operation at higher benzene/olefin molar nor reversed by regenerating the catalyst with benzene. Catalyst deactivation that cannot be reversed by typical regeneration procedures is sometimes referred to as “permanent,” either because it sometimes requires additional reactivation measures beyond contacting the catalyst with benzene or because it sometimes requires shutting down and replacing the catalyst.
Methods are sought to minimize deactivation of the alkylation catalyst.